Method for laser-processing semiconductor device

ABSTRACT

A linear laser light which has an energy and is to be scanned is irradiated to a semiconductor device formed on a substrate, and then the substrate is rotated to irradiate to the semiconductor device a linear laser light which has a higher energy than that of the irradiated linear laser light and is to be scanned. Also, in a semiconductor device having an analog circuit region and a remaining circuit region wherein the analog circuit region is smaller than the remaining circuit region, a linear laser light having an irradiation area is irradiated to the analog circuit region without moving the irradiation area so as not to overlap the laser lights by scanning. On the other hand, the linear laser light to be scanned is irradiated to the remaining circuit region.

This application is a Divisional application of Ser. No. 08/641,695,filed May 2, 1996, now U.S. Pat. No. 6,096,581, which is a continuationof application of Ser. No. 08/400,867, filed Mar. 8, 1995, nowabandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a method for laser light irradiation(so called laser annealing) in fabrication of a semiconductor device,and to a method for fabricating a semiconductor device by laser lightirradiation, wherein mass production is high, characteristics are littledifferent among the devices, and a production yield is high. Moreparticularly, the invention relates to a method for improving orrecovering (repairing) crystallinity of a semiconductor material byirradiating the material with laser light. The semiconductor materialincludes a semiconductor material having wholly or partially amorphouscomponents, a substantially intrinsic polycrystalline semiconductormaterial, and a semiconductor material whose crystallinity has beenseverely deteriorated by damage due to ion irradiation, ionimplantation, or ion doping.

Recently, researches have been conducted concerning low temperaturesemiconductor device fabrication processes mainly because it has becomenecessary to form semiconductor devices on an insulating substrate madeof glass or the like. Also, miniaturization of devices and making of amultilayer structure have required.

In semiconductor fabrication processes, it may be necessary tocrystallize an amorphous component contained in a semiconductor materialor an amorphous semiconductor material. Also, it may be necessary torecover (repair) the crystallinity of a semiconductor materialdeteriorated by ion irradiation. Furthermore, when there existscrystallinity, it may be required to be enhanced. In general, thermalannealing is used for these purposes. When silicon is used as asemiconductor material, it is annealed at 600 to 1100° C. for 0.1 to 48hours or longer. As a result, the amorphous component is crystallized,the crystallinity recovered, or the crystallinity is improved.

In thermal annealing, as higher temperature is used, the processing timecan be shortened. However, at 500° C. or lower, almost no effectproduces. At about 600° C., a long processing time is needed.Accordingly, it has been required that the thermal annealing be replacedby other means in order to lower the process temperature. Hence, a laserirradiation technique has attracted attention as an ultimate lowtemperature process. Since laser light can be irradiated only onto aregion that needs high energy corresponding to the energy of thermalannealing, it is not necessary that the whole substrate be processed toa high temperature.

Generally, two methods have been proposed to irradiate laser light.First method uses a continuous oscillating laser such as an argon ionlaser. The laser beam having a spot shape is irradiated to asemiconductor material. In this method, the semiconductor material ismelted by variations in the energy distribution inside the beam and bymovement of the beam, and then the material is solidified mildly. As aresult, the semiconductor material is crystallized. Second methodemploys a pulse oscillating laser such as an excimer laser. The pulselaser having high energy is irradiated to a semiconductor material. Inthis method, the material is momentarily melted and solidified, wherebythe material is crystallized.

The first method has the problem that it takes a long time to performthe processing, for the following reason. Since the maximum energy of acontinuous oscillating laser is limited, the maximum beam spot size isseveral millimeters. On the other hand, in the second method, a largespot of several cm² or more can be used because the maximum energy isvery large. Hence, the productivity can be improved.

However, in order to process one large area substrate with a normallyused beam having a square or rectangle form, it is necessary to scan(move) the beam vertically and horizontally. This produces limitationson the productivity. This problem can be solved by modifying the crosssection of the beam into a linear form, making the width of the beamlarger than the size of the substrate to be processed, and scanning thisbeam.

The remaining problem for improvement is uniformity of the effect oflaser irradiation. A pulse laser somewhat varies in energy from pulse topulse and so it is difficult to uniformly process the whole substrate.Especially, it is important to obtain uniform the characteristics ofregions where adjacent laser spots overlap each other.

Also, when a pulse laser is irradiated, even if the uniformity of theenergy inside the beam of one shot pulse can be accomplished byimprovements in the optical system, it is difficult to reduce variationsin the characteristics of devices due to overlap of pulse laser.Especially, where devices are located just at ends of the beam of laserlight, the characteristics (especially the threshold voltages of MOStransistors) vary considerably from device to device.

In semiconductor devices, considerable variations in the thresholdvoltages of digital circuits are admitted. In analog circuits, thedifference between the threshold voltages of adjacent transistors isrequired to be 0.02 V or less.

It has been reported that if weak pulse laser light is preliminarilyirradiated before irradiation of strong pulse laser light, thenonuniformity is lowered and the uniformity is improved. However,overlap of laser spots has not been discussed sufficiently.

SUMMARY OF THE INVENTION

The object of the present invention is to solve the above problem byusing a linear laser beam (linear laser light). That is, a relativelyweak, first pulse linear laser light is irradiated to a substrate. Then,a second linear laser light having higher energy is irradiated at rightangles to the first laser light to process the substrate. Absoluteoutputs of the first laser light and the second laser light may bedetermined by the required uniformity and characteristics.

In order that the first linear laser light make substantially rightangles with respect to the second laser light, the direction of eitherlaser light is varied, or the substrate is rotated at about a ¼revolution (approximately 90°), generally n/2+¼ (n is 0, 1, 2, . . . )revolution, i.e., (n/2+¼)×360°. A basic embodiment is shown in FIGS.1A-1C. A substrate of a rectangle ABCD is disposed as shown in FIG. 1A.A linear laser light 1 is scanned in the direction indicated by thearrow, i.e., from top to bottom, to process the substrate. It is assumedthat the laser light output has a relatively weak energy. In the region2 (indicated by the broken lines) irradiated with the laser light,nonuniformity by variations in pulse intensity of the laser light andoverlap of laser spots may be observed. A region 3 is not yet irradiatedwith the laser light.

Then, the substrate is rotated at a ¼ revolution, i.e., 90° (FIG. 1B).The laser light 4 is scanned again in the direction indicated by thearrow, or from top to bottom, to process the substrate. At this time,the laser light output is larger than the laser light output irradiatedfirst (FIG. 1C).

From the above description, in the present invention, the direction ofthe nonuniformity due to second laser irradiation is perpendicular tothat the nonuniformity due to the first laser irradiation. Therefore,since these two kinds of nonuniformities cancel out, semiconductordevices having high uniformity can be obtained.

The present invention can be applied to, as an object irradiated with alaser light, a film having no pattern, or substantially a completeddevice. Since the present invention is characterized in that two linearlaser beams are used substantially in an orthogonal relation to eachother and irradiated to the object at least twice, the laser light canbe used less wastefully for square and rectangular substrates than forcircular substrates. In the present invention, a circular substrate canbe processed. In the invention, some patterns are available, dependingon configurations of circuits formed on a substrate to be processed.

Also, it is desired to irradiate a laser light which is large enough tocover the Whole circuit simultaneously, to prevent variations by overlapof laser beams. In practice, however, this is impossible to achieve. Inthe present invention, a relatively narrow region in which the laserbeams do not overlap each other and a relatively wide region in whichthe laser beams overlap each other are formed on a substrate, to obtainsufficient characteristics as a whole.

In the present invention, the circuits formed on the substrate aredivided into a first circuit region having mainly an analog circuit anda second circuit region which is less closely associated with analogelements. The beam size of the laser light is larger than the firstcircuit region. In this way, the first circuit region can be totallyirradiated with the laser light substantially without moving the laserlight.

In the first circuit region, the laser light is irradiated withoutsubstantially moving the laser light. Therefore, in the first circuitregion, overlap of the laser beam do not produce. On the other hand, inthe second circuit region, the laser light is irradiated while scanningthe laser light. As a result, the laser beams overlap with each other.

In a monolithic liquid crystal display device which both an activematrix circuit and a peripheral circuit (driver circuit) for driving theactive matrix circuit are formed on the same substrate, the firstcircuit region including mainly analog circuits corresponds to thedriver circuit for driving the active matrix circuit, especially asource driver (column driver) circuit for outputting an analog signal.The second circuit region less closely associated with analog elementscorresponds to the active matrix circuit and to a gate driver (scandriver) circuit.

In the present invention, it is necessary to match the shape of thelaser beam to such circuits or to match the shapes of the circuits tothe laser beam. Generally, it is desired to use the laser beam having alinear or rectangular form. In the liquid crystal display device, sincethe column driver and the scan driver are formed substantially in aperpendicular relation to each other, the direction of the laser lightis varied, or the substrate is rotated at about a ¼ revolution,approximately 90°, generally n/2+¼ (n is 0, 1, 2, . . . ) revolution,i.e., (n/2+¼)×360° as described above.

By the above processing, in the first analog circuit region, sinceoverlap of the laser beam do not produce, the uniformity of the laserbeam within its plane (in-plane uniformity) is important. Consequently,devices having uniform characteristics can be formed by sufficientlyimproving the in-plane uniformity of the laser beam. On the other hand,in the second circuit region, variations in characteristics areinevitably caused by overlap of the laser beams. However, slightvariations are admitted essentially in such a circuit. Hence, no greatproblems produce.

In this manner, in the present invention, the whole circuit formed onthe substrate is prevented from being affected by overlap of the laserbeams. In consequence, the characteristics of the whole circuit areimproved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C are views illustrating the concept of the present invention;

FIGS. 2A-2D are views illustrating a laser-processing method accordingto the invention;

FIG. 3 is a diagram illustrating TFTs forming analog switches of acolumn driver in an embodiment of the invention;

FIGS. 4A-4E are views illustrating another laser processing methodaccording to the invention;

FIG. 5 is a perspective structure view of a laser annealing apparatusused in the invention;

FIGS. 6A-6C are optical system diagrams in the laser annealing apparatusshown in FIG. 5;

FIGS. 7A-7E are views illustrating another laser processing methodaccording to the invention;

FIGS. 8A-8G are views illustrating another laser processing methodaccording to the invention;

FIGS. 9A-9G are views illustrating another laser processing methodaccording to the invention;

FIGS. 10A-10F are cross sections schematically illustratingmanufacturing processes of TFTs according to the invention; and

FIG. 11 is a cross-sectional view of a TFT circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS EMBODIMENT 1

FIG. 5 is a perspective view of a laser annealing apparatus used in thepresent invention. Laser light is generated by a resonator (oscillator)52 and input to an amplifier 53 via total reflection mirrors 55 and 56,to amplify the laser light. The amplified laser light is introduced intoan optical system 54 via total reflection mirrors 57 and 58. The laserlight is a rectangle shape having approximately 3×2 cm² but the laserlight is processed (focused) by the optical system 54 into a linear beamhaving a length of 10 to 30 cm and a width of 0.3 cm. The maximum energyof the laser light produced from the optical system 54 is 1000 mJ pershot.

The optical path inside the optical system 54 is shown in FIGS. 6A-6C.FIG. 6A is an upper view, FIG. 6B is a side view and FIG. 6C is aperspective view. The laser light incident on the optical system 54passes through a cylindrical concave lens A, a cylindrical convex lensB, a lateral flyeye lens C, and a longitudinal flyeye lens D. Since thelight passes through the flyeye lenses C and D, the energy distributionof the laser light changes from a gaussian distribution to a squaredistribution. The laser light also passes through cylindrical convexlenses E and F and is focused onto a cylindrical lens H via a mirror G(mirror 59 in FIG. 5). Then, the focused laser light is irradiated to asample.

In the present embodiment, distances X1 and X2 are constant. Thedistance X3 between a virtual focus I (arising because the flyeye lensesdiffer in curvature) and the mirror G and distances X4 and X5 areadjusted by magnification M and focal distance F. That is, among thesefactors, there exist relations given by

M=(X₃+X₄)/X₅

1/F=1/(X₃+X₁)+1/X₅

The total optical path length X6 is about 1.3 m.

The use of the linear laser light greatly improves the throughput. Inparticular, the linear laser beam output from the optical system 54 isirradiated to a sample 61 via the total reflection mirror 59. Since thewidth of the laser beam is equal to or greater than the width of thesample 61, the sample 61 may be moved only in one direction. A stage anda driver 60 for the sample 61 are simple in structure and can be easilymaintained. Also, when the sample 61 is placed, it can be readilyaligned. In the present invention, the sample 61 may be rotated inaddition to moving it in one direction.

On the other hand, since it is impossible for the laser radiation by thelaser light having approximately square shape to irradiate the wholesubstrate, it is necessary to move the sample 61 both vertically andhorizontally, i.e., in two dimensions. However, this complicates thestructure of the driver 60. Also, the alignment must be made in twodimensions and so this is difficult to accomplish. Especially, where thealignment is made manually, time is consumed wastefully during thisprocess. Hence, the productivity deteriorates. It is necessary to holdthese devices on a stable pedestal 51 such as a vibration-proof support.

The present embodiment relates to a monolithic active matrix liquidcrystal display (AMLCD) device where an active matrix circuit and aperipheral circuit for driving the matrix circuit are formed on the samesubstrate. Thin film transistors (TFTs) are used in the AMLCD device.The process for fabricating these TFTs is roughly as follows.

(1) A silicon oxide film is formed as a base layer on a glass substrate.An amorphous silicon film is formed on the silicon oxide film. An agentfor promoting crystallization such as nickel acetate is applied to thetop surface of the amorphous silicon film.

(2) The amorphous silicon film is crystallized by solid phase growth,for example, at 550° C. for 8 hours in a nitrogen atmosphere.

(3) The crystallized silicon film is processed with laser radiation inorder to improve the crystallinity.

(4) The silicon film is etched to form an island silicon region.

(5) A gate insulating film (silicon oxide) is formed.

(6) Gate electrodes are formed.

(7) An impurity such as phosphorus or boron is implanted to form sourceand drain.

(8) The implanted impurity is activated by laser irradiation.

(9) An interlayer insulator is formed.

(10) Electrodes are formed on the source and drain.

The present embodiment relates to the above-described laser irradiationprocess (3) performed to further improve the crystallinity of thepolycrystalline silicon film. The present embodiment is illustrated inFIGS. 2A-2D. As shown in FIG. 2A, in the AMLCD device, an active matrixcircuit 22, a column driver 23, and a scan driver 24 are formed on asubstrate 21. The column driver 23 and the scan driver 24 are similar incircuit configuration. Generally, a large number of TFTs are formed in alongitudinal direction of the driver region, as shown in FIG. 3representing analog switch TFTs for the column driver. Each TFT has alarge channel width of about 800 μm.

As shown in FIG. 2B, a linear laser light 25 is scanned in the directionindicated by the arrow (i.e., from top to bottom as viewed in thefigure) and irradiated at a substrate temperature of 200° C. in theatmosphere pressure. The laser light is a KrF excimer laser (having awavelength of 248 nm, an oscillation frequency of 10 Hz and an energydensity of 200 mJ/cm2) and scanned at a scanning speed of 3 mm/sec.Therefore, the laser light is moved in steps of 300 μm in the directionindicated by the arrow. Since the width of the laser beam is 0.3 mm,about 10 shots of the laser light are irradiated to each location.

Thereafter, the substrate is rotated through 90° in a clockwisedirection (FIG. 2C). The laser light is again scanned in the directionindicated by the arrow to perform the laser irradiation process at asubstrate temperature of 200° C. in the atmosphere pressure (FIG. 2D).This laser irradiation differs from the previous laser irradiation onlyin that the energy density is 300 mJ/cm².

Since the energy density of the second laser irradiation is greater thanthat of the first laser irradiation, the characteristics of thesemiconductor device are affected materially. Although the presentinvention considerably reduces laser energy variations, completesuppression is not yet achieved. In FIG. 3, in a column driver, laserlights overlap each other on a region indicated by the dot-and-dash lineas a result of the first laser irradiation. Then, the laser lights movelongitudinally of the column driver and so the laser lights overlap eachother in the region indicated by the dotted line.

The TFTs are especially affected greatly, because the energy density ofthe second laser irradiation is large. Therefore, the characteristics ofadjacent TFTs may be affected greatly by variations in energy densityamong each shot in the laser radiation. In practice, the variations canbe suppressed sufficiently by preliminary laser irradiation. In acircuit having analog switches such as the column driver, a differenceof the threshold voltages between adjacent TFTs should not be 0.02 V ormore.

For this reason, with the method of FIGS. 2A-2D, there is thepossibility that the TFTs of the column driver differ greatly inthreshold voltage. Accordingly, a method of FIGS. 4A-4E is used.

A substrate 31 on which an active matrix circuit 32, a column driver 33,and a scan driver 34 are formed is placed in the same arrangement as inFIG. 2A (FIG. 4A) and rotated at 90° in a clockwise direction (FIG. 4B).

Then, laser light 35 is scanned in the direction indicated by the arrow(from top to bottom-as viewed in the figure) to perform laser processingat a substrate temperature of 200° C. in the atmosphere pressure (FIG.4C). The laser light is irradiated under the same conditions as in thecase already described in FIGS. 2A-2D. That is, the oscillationfrequency is 10 Hz. The energy density is 200 mJ/cm2. The scanning speedis 3 mm/sec. Then, the substrate 31 is rotated in a counterclockwisedirection (FIG. 4D).

The laser light 35 is scanned in the direction indicated by the arrow toperform laser processing (FIG. 4E). The laser irradiation condition isthe same condition as the previous laser irradiation condition exceptthat the laser energy density is 300 mJ/cm².

In this method, the first laser irradiation produces an overlap on theregion indicated by the dotted line in FIG. 3 on the column driver 33.When driver intenser laser light is irradiated, an overlap is producedin the region indicated by the dot-and-dash line in FIG. 3. Inconsequence, variations in the characteristics of the TFTs aresuppressed greatly.

The scan driver 34 may suffer from the same problems as the problemswith respect to the column driver 33 in the method of FIGS. 2A-2D.However, no analog switches are formed in the scan driver 34, unlike thecolumn driver 33. It suffices to suppress the variations in thethreshold voltages of the adjacent TFTs to about 0.1 V. This degree ofvariations can be accomplished, for example, by the method illustratedin FIGS. 1A-1C.

In this way, the present invention can be expanded further to improvethe uniformity of semiconductor devices. Although the present embodimentrelates to improvements in the crystallinity by laser irradiation, theprocess (8) for activating source and drain regions after introductionof a dopant can be carried out similarly.

EMBODIMENT 2

In the present embodiment, the laser annealing apparatus of FIG. 5 isused. The laser light incident on an optical system 54 is a rectangleshape having approximately 3×90 mm² but the laser light is processed(focused) by the optical system 54 into a linear laser beam having alength of 100 to 300 mm and a width of 10 to 30 mm. The maximum energyof the laser light passed through the optical system 54 is 30 J pershot.

The above laser annealing apparatus may be used alone or in combinationwith other apparatus such as a plasma CVD film deposition apparatus, anion implantation apparatus (ion doping apparatus), a thermal annealingapparatus, or other semiconductor fabrication apparatus. Thiscombination is known as a multi-chamber system.

The present embodiment relates to a monolithic AMLCD device. In thisdevice, an active matrix circuit, a column driver, and a scan driver areformed on a substrate, as described above. In practice, when a laserirradiation is conducted, only a uniform film is formed on thesubstrate. The column driver and the scan driver have shift registers.Since the column driver outputs an anal)g signal, the column driverfurther includes an amplifier (buffer circuit).

The TFTs used in the AMLCD device are manufactured as summarilydescribed above. The present and subsequent embodiments relate to laserirradiation process (3) for further improving the crystallinity ofpolycrystalline silicon film.

FIGS. 7A-7E show laser processes in the present embodiment. As shown inFIG. 7A, a laser beam irradiation region 215 is a rectangular shapehaving a width of 10 mm and a length of 300 mm and is large enough toirradiate a whole column driver 213. A substrate 211 is moved toirradiate the laser light to the column driver 213. At this stage, thelaser light is not irradiated to the substrate 211. Subsequently, thesubstrate 211 is irradiated with the laser light at a substratetemperature of 200° C. in the atmosphere pressure in such a way that thelaser beam and the substrate 211 are hardly moved. The laser light is aKrF excimer laser having a wavelength of 248 nm, an oscillationfrequency of 10 Hz, and an energy density of 300 mJ/cm². The number ofthe laser pulses is 10 shots. After the required number of shots in thelaser light is irradiated, the laser irradiation is stopped (FIG. 7B).

Then, the substrate 211 is moved from top to bottom as viewed in FIG. 7Cup to the position where the irradiation region 215 overlaps the upperportions of an active matrix region 214 and a scan driver 212 (FIG. 7C).Thereafter, the substrate 211 is moved while irradiating the laser light(FIG. 7D). The oscillation frequency is 10 Hz. The energy density is 300mJ/cm2. The scanning speed is 10 mm/sec. Therefore, the irradiationregion 215 moves in steps of 1 mm. Since the width of the laser light is10 mm, about 10 shots in the laser radiation are irradiated to eachlocation.

The laser light is scanned to the lower end of the substrate 211, sothat the scan driver 212 and the active matrix region 214 are irradiatedwith the laser light (FIG. 7E).

In the present embodiment, overlap of the laser beam do not produce onthe column driver 213. As a result, the TFTs in the column driver 213little differ from each other in threshold voltage. Typically, thedifference between the threshold voltages of adjacent TFTs is 0.01 V orless. Variation in threshold voltage within the column driver 213 is0.05 V or less. Similarly, other characteristics differ only a littleamong the TFTs. On the other hand, adjacent laser beams overlap eachother on the scan driver 212 and the active matrix region 214.Consequently, the difference between the threshold voltages of theadjacent TFTs in the scan driver 212 is about 0.1 V. Variation inthreshold voltage within the scan driver 212 is similar value. In theactive matrix region 214, a similar value is obtained. Such variationsdo not hinder the operation of these circuits.

The column driver may be irradiated with the laser light after the scandriver 212 and the active matrix region 214.

EMBODIMENT 3

Laser processing of the present embodiment are shown in FIGS. 8A-8G. InFIG. 8A, a laser beam irradiation region 225 is a rectangular form andis large enough to irradiate a whole column driver 223. The rectangularform has 10 mm wide and 200 mm long. A substrate 221 is moved toirradiate the laser light to the column driver 123. At this stage, thelaser light is not irradiated to the substrate 221. Subsequently, thesubstrate 221 is irradiated with the laser light at a substratetemperature of 200° C. in the atmosphere pressure in such a way that thelaser beam and the substrate are hardly moved. The laser light is a KrFexcimer laser having a wavelength of 248 nm, an oscillation frequency of10 Hz, and an energy density of 300 mj/cm2. The number of laser pulsesis ten shots. After the required number of shots in the laser light areirradiated, the laser irradiation is stopped (FIG. 8B).

Then, the substrate 221 is move from top to bottom as viewed in FIG. 8Cuntil the irradiation region 225 overlaps the upper end of the activematrix region 224. Unlike Embodiment 2, the scan driver 222 is notirradiated with the laser light.

Thereafter, the substrate 221 is moved while irradiated with the laserlight (FIG. 8D). The oscillation frequency is 10 Hz. The energy densityis 250 mJ/cm2. The scanning speed is 10 mm/sec. Therefore, theirradiation region 225 moves in steps of 1 mm. Since the width of laserlight is 10 mm, about 10 shots in the laser light are irradiated to eachlocation.

In this way, the laser light is scanned until the lower end of thesubstrate, so that the active matrix region 224 is irradiated with thelaser light (FIG. 8E).

Then, the substrate 221 is rotated at a quarter revolution (FIG. 8F). InFIG. 8F, a square 226 indicated by the dotted line is an initialposition of the substrate 221. As shown in FIG. 8G, the substrate 221 ismoved to irradiate the laser light to the scan driver 222. At thisstage, the laser light is not irradiated to the substrate 221.Thereafter, the scan driver 222 is irradiated with laser light in such away that the laser beam and the substrate are hardly moved. Theoscillation frequency is 10 Hz. The energy density is 300 mJ/cm2. Thenumber of laser pulses is 10 shots. After the required number of shotsof the laser light are irradiated, the laser irradiation is stopped.

In the present embodiment, overlap of the laser beam do not produce onthe scan driver 222, as well as on the column driver 223. Also in thepresent embodiment, the driver circuit is irradiated with laserradiation of 300 mJ/cm2 but the active matrix circuit is irradiated withlaser radiation of 250 mJ/cm2 in order to obtain TFTs which exhibitsmall leakage current (OFF current) when a reverse bias voltage isapplied to each gate electrode. On the other hand, TFTs of the drivercircuit are required to operate at high speeds. Hence, a high mobilityis obtained by making the energy of the laser light high.

The scan driver 22 may be irradiated with the laser light afterirradiations of the column driver 223 and the active matrix region 224and rotation of the substrate 221.

EMBODIMENT 4

Laser processes of the present embodiment are illustrated in FIGS.9A-9G. The present embodiment relates to a monolithic liquid crystaldisplay (LAD) having driver circuits on the upper and lower portions andon the right and left side portions of an active matrix (circuit)region, unlike Embodiments 2 and 3. The present embodiment also relatesto a activating process for this display device. This activating stepcorresponds to activation (process (8)) of the dopant implanted by laserirradiation in Embodiment 1.

FIGS. 10A-10F illustrate the sequence for processing the whole substrateaccording to the present embodiment. The substrate 101 is made ofCorning 7059 and has 300 mm×200 mm in size. A silicon oxide film isformed as a base oxide film 102 at a thickness of 1000 to 5000 Å, forexample, 2000 Å, on the substrate 101 by sputtering in an oxygenatmosphere. However, in order to enhance the productivity, the siliconoxide film may be formed by decomposing TEOS by PCVD. Furthermore, theformed silicon oxide film may be annealed at 400 to 650° C.

Then, an amorphous silicon film having a thickness of 300 to 5000 Å,preferably 400 to 1000 Å, for example, 500 Å, is deposited by plasma CVDor LPCVD. The laminate is allowed to stand for 8 to 24 hours in areducing atmosphere at 550 to 600° C. to crystallize the amorphoussilicon film. At this time, a trace amount of a metal element such asnickel which promotes the crystallization may be added. Also, thisprocess may be carried out using laser irradiation. The crystallizedsilicon film is etched into an island region 103. Then, a silicon oxidefilm 104 having a thickness of 700 to 1500 Å, for example, 1200 Å, isformed by plasma CVD.

Subsequently, an aluminum film having a thickness of 1000 Å to 3 μm, forexample, 5000 Å, and containing 1% by weight of Si or 0.1 to 0.3% byweight of Sc (scandium) is formed by sputtering. The laminate is etchedto form a gate electrode 105 and a gate interconnect (wiring) 106 (FIG.10A).

A current is passed through a gate electrode 105 and through a gateinterconnect 106 within an electrolytic solution to perform anodicoxidation. Thus, anodic oxides 107 and 108 each having a thickness of500 to 2500 Å, for example, 2000 Å, are formed. The electrolyticsolution includes L-tartaric acid diluted with ethylene glycol at aconcentration of 5%. The pH of this solution is adjusted to 7.0±0.2,using ammonia. The substrate 101 is immersed in this solution. Thepositive terminal of a constant current source is connected with thegate interconnect on the substrate, while the negative terminal isconnected with a platinum electrode. A voltage is applied at a constantcurrent of 20 mA. The oxidation is continued until the voltage reaches150 V. Furthermore, the oxidation is continued at a constant voltage of150 V until the current drops below 0.1 mA. As a result, an aluminumoxide film having a thickness of 2000 Å is obtained.

Then, using the gate electrode portion (or, gate electrode and itssurrounding anodic oxide film) as a mask, a dopant, or phosphorus, isimplanted into the island region (silicon film) 103 in self-aligning byion doping. As a result, as shown in FIG. 10B, lightly doped drain (LDD)regions 109 are formed. The dose is 1×10¹³ to 5×10¹⁴ atoms/cm², forexample, 5×10¹³ atoms/cm². The accelerating voltage is 10 to 90 kV, forexample, 80 kV (FIG. 10B).

Thereafter, a silicon oxide film 110 is deposited by plasma CVD. In thepresent embodiment, TEOS and oxygen are used as raw gases.Alternatively, monosilane and dinitrogen monoxide are employed. Theoptimum value of the thickness of the silicon oxide film 110 variesdepending on tile height of the gate electrode and the gateinterconnect. In the present embodiment, the height including the anodicoxide film is about 6000 Å. In this case, the thickness of the siliconoxide film 110 is preferably one third to 2 times this value, forexample, 2000 Å to 1.2 μm. In this embodiment, the thickness is set to6000 Å. In this film formation process, the film thickness in planarportions must be uniform. Also, good step coverage is required. As aresult, the thickness of the silicon oxide film on the side surfaces ofthe gate electrode and the gate interconnect is increased by the portionindicated by the dotted lines in FIG. 10C.

Then, the silicon oxide film 108 is etched by anisotropic dry etchingbased on well known reactive ion etching (RIE) techniques. This etchingprocess ends when the etched region arrives at the gate insulating film105. The instant at which the etching process ends can be controlled by,for example, making the etching rate of the gate insulating film 105smaller than that of the silicon oxide film 110. In consequence, roughlytriangular insulators, or side walls 111 and 112, remains on the sidesurfaces of the gate electrode and the gate interconnect.

Phosphorus is again introduced by ion doping. The dose is preferably 1to 3 orders of concentration greater than the dose used in the processillustrated in FIG. 10B. In the present embodiment, the dose is 2×10¹⁵atoms/cm², which is 40 times as great as the dose used in the firstdoping of phosphorus. The accelerating voltage is 80 kV. As a result,heavily phosphorus-doped regions (source and drain) 114 into whichphosphorus having a high concentration is introduced are formed. Lightlydoped drain (LDD) regions 113 remains under the side walls 111 and 112(FIG. 10D).

A KrF excimer laser having a wavelength of 248 nm and a pulse width of20 ns is irradiated to activate the doped impurity. The energy densityis 200 to 400 mJ/cm², preferably 250 to 300 mJ/cm² (FIG. 10E)

A silicon oxide film is formed as an interlayer insulator 115 having athickness of 5000 Å over the whole surface of the laminate by CVD.Contact holes are formed in the source and drain regions 114. Secondlayer aluminum interconnect and electrode 116 and 117 is formed. Thethickness of the aluminum interconnect is substantially equal to thethickness of the gate electrode and gate interconnect and 4000 to 6000Å.

In this way, TFTs having N-channel LDDs are completed. To activate theimpurity regions, hydrogen annealing may be performed at 200 to 400° C.The second layer interconnect 117 gets over the gate interconnects 106,thus forming a step. This step is made milder by the presence of theside wall 112. Therefore, little steep step edges are observed althoughthe second layer interconnect is substantially equal in thickness to thegate electrode and interconnect (FIG. 10F).

Of the above processes, the process for activating the dopant (impurity)by laser irradiation in FIG. 10E will be described.

FIG. 11 shows a cross section of a substrate processed in the presentembodiment. A peripheral driver circuit region and a pixel circuitregion are formed on the substrate. The peripheral driver circuit regionhas NMOS TFTs and PMOS TFTs. The pixel circuit has PMOS TFTs which areconnected with pixel electrodes.

FIG. 9A is a top view of the substrate to be processed in the presentembodiment. In FIG. 10E, an interlayer insulator and a second layerinterconnect are not formed. As shown in FIG. 9A, scan drivers 232 and233, column drivers 234 and 235, and an active matrix circuit 236 areformed on a substrate 231. A laser beam irradiation region 237 has arectangular form and is large enough to irradiate the whole columndrivers 234 and 235. The rectangular form is 10 mm wide and 300 mm long.

In FIG. 9B, the substrate 231 is moved to irradiate the laser light tothe scan driver 232. At this stage, the laser light is not irradiated tothe substrate 231. Thereafter, the laser radiation is performed at asubstrate temperature of 200° C. in an atmosphere pressure in such a waythat the laser beam and the substrate 231 are hardly moved. The laserlight is a KrF excimer laser having a wavelength of 248 nm, anoscillation frequency of 10 Hz, and an energy density of 300 mJ/cm2. Thenumber of pulses in the laser beam is 10 shots. After the requirednumber of shots are irradiated, the laser irradiation is stopped.

Thereafter, the substrate 231 is moved to irradiate the laser light tothe scan driver 233. 10 shots of the laser light are irradiated underthe same conditions as the above described conditions without moving thesubstrate 231 and the laser beam (corresponding to the laser beamirradiation region). After a required number of shots are irradiated,the laser irradiation is stopped (FIG. 9C).

Then, the substrate 231 is rotated at a quarter revolution (FIG. 9D). Asquare 238 indicated by the dotted line is an initial position of thesubstrate 231. In FIG. 9E, the substrate 231 is moved to irradiate thelaser light to the column driver 234. Then, 10 shots of the laser lightare irradiated under the same conditions as the above describedconditions without moving the substrate 231 and the laser beam. After arequired number of shots of laser light are irradiated, the laserirradiation is stopped (FIG. 9E).

Thereafter, the substrate 231 is moved from top to bottom as viewed inFIG. 9F to the position where the irradiation region 237 overlaps upperportions of the active matrix circuit 236 and the scan drivers 232, 233.Then, the irradiation region 237 is moved in the direction indicated bythe arrow while irradiated with the laser radiation. The oscillationfrequency is 10 Hz. The energy density is 250 mJ/cm2. The scanning speedis 10 mm/sec. Consequently, the irradiation region 237 moves in steps of1 mm. Since the width of the laser light is 10 mm, about 10 shots of thelaser light are irradiated to each location.

The laser light is scanned to the lower end of the active matrix circuit236 and then the laser irradiation is stopped.

As shown in FIG. 10G, the substrate 231 is moved to irradiate the laserbeam to the column driver 235. Then, the column driver 235 is irradiatedwith the laser light without moving the laser beam and the substrate231. The oscillation frequency is 10 Hz. The energy density is 300mJ/cm2. The number of shots of the laser light is 10 shots. After arequired number of shots of laser light are irradiated, the laserirradiation is stopped.

In the present embodiment, overlap of the laser beam do not produce inthe column driver. On the other hand, in the scan driver, the laserbeams do not overlap in the laser irradiation processes shown in FIGS.9B and 9C but overlaps occur when the active matrix circuit isirradiated with the laser beam. However, admissible variations incharacteristics of the TFTs forming the scan driver is larger than thatin the TFTs forming the column driver. Also, since the energy of thelaser light irradiated to the active matrix circuit is smaller than theenergy of the first laser irradiation, substantially effects do notproduce.

The laser irradiation techniques of the present invention improve,theuniformity of semiconductor devices while maintaining the productivity.The present invention can be applied to every laser process used toprocess semiconductor devices. Especially, where TFTs are used assemiconductor devices, the uniformity of the threshold voltages of theTFTs can be effectively enhanced by irradiating the polycrystallinesilicon film with laser light. Furthermore, where the invention isapplied to activation of the impurity element in the source and drainand the above process is also carried out, the field mobilities of theTFTs or the uniformity of the ON currents can be effectively improved.In this way, the present invention is useful for industry.

What is claimed is:
 1. A method of manufacturing a display device havingan active matrix circuit and a driver circuit both formed over a samesubstrate, said driver circuit including at least one CMOS devicecomprising an N-channel type thin film transistor and a P-channel typethin film transistor, said method comprising the steps of: forming asemiconductor film comprising amorphous silicon over said substrate;crystallizing said semiconductor film by heating; irradiating saidsemiconductor film with a laser light after said crystallizing stepwherein an irradiation area of said laser light on said semiconductorfilm is elongated in one direction; etching the crystallizedsemiconductor film after the irradiating step into at least onesemiconductor island; and introducing an N-type impurity into portionsof said semiconductor island to form source and drain regions and atleast one lightly doped region for said N-channel type thin filmtransistor, wherein said P-channel type thin film transistor has nolightly doped region.
 2. A method according to claim 1 wherein thecrystallization of said semiconductor film is promoted by adding a metalelement.
 3. A method according to claim 1 further comprising a step ofannealing the semiconductor island in a hydrogen atmosphere after theintroduction of the N-type impurity.
 4. A method according to claim 1wherein said laser is a pulsed excimer laser.
 5. A method according toclaim 3 wherein a dose amount of said lightly doped region is 1×10¹³ to5×10¹⁴ atoms/cm².
 6. A method according to claim 1 wherein saidirradiating step is performed without moving said laser light at aregion of said driver circuit.
 7. A method according to claim 1 whereinsaid irradiating step is performed so that one portion of saidsemiconductor film is irradiated with 10 pulses of said laser light. 8.A method of manufacturing a display device having an active matrixcircuit and a driver circuit both formed over a same substrate, saiddriver circuit including at least one CMOS device comprising anN-channel type thin film transistor and a P-channel type thin filmtransistor, said method comprising the steps of: forming a semiconductorfilm comprising amorphous silicon over said substrate; crystallizingsaid semiconductor film by heating; irradiating said semiconductor filmwith a laser light after said crystallizing step wherein an irradiationarea of said laser light on said semiconductor film is elongated in onedirection; etching the crystallized semiconductor film after theirradiating step into at least one semiconductor island; and introducingan N-type impurity into portions of said semiconductor island to formsource and drain regions and at least one lightly doped region for saidN-channel type thin film transistor, wherein said lightly doped regionis formed only in said N-channel type thin film transistor.
 9. A methodaccording to claim 8 wherein the crystallization of said semiconductorfilm is promoted by adding a metal element.
 10. A method according toclaim 8 further comprising a step of annealing the semiconductor islandin a hydrogen atmosphere after the introduction of the N-type impurity.11. A method according to claim 8 wherein said laser is a pulsed excimerlaser.
 12. A method according to claim 8 wherein a dose amount of saidlightly doped region is 1×10¹³ to 5×10¹⁴ atoms/cm².
 13. A methodaccording to claim 8 wherein said irradiating step is performed withoutmoving said laser light at a region of said driver circuit.
 14. A methodaccording to claim 8 wherein said irradiating step is performed so thatone portion of said semiconductor film is irradiated with 10 pulses ofsaid laser light.
 15. A method of manufacturing a display device havingan active matrix circuit and a driver circuit both formed over a samesubstrate, said driver circuit including at least one CMOS devicecomprising an N-channel type thin film transistor and a P-channel typethin film transistor, said method comprising the steps of: forming asemiconductor film comprising amorphous silicon over said substrate;crystallizing said semiconductor film by heating; irradiating saidsemiconductor film with a laser light after said crystallizing stepwherein an irradiation area of said laser light on said semiconductorfilm is elongated in one direction; etching the crystallizedsemiconductor film after the irradiating step into at least onesemiconductor island; introducing an N-type impurity into portions ofsaid semiconductor island to form source and drain regions and at leastone lightly doped region for said N-channel type thin film transistor;and forming an interlayer insulating film over said semiconductorisland, wherein said P-channel type thin film transistor has no lightlydoped region.
 16. A method according to claim 15 wherein thecrystallization of said semiconductor film is promoted by adding a metalelement.
 17. A method according to claim 15 further comprising a step ofannealing the semiconductor island in a hydrogen atmosphere after theintroduction of the N-type impurity.
 18. A method according to claim 15wherein said laser is a pulsed excimer laser.
 19. A method according toclaim 15 wherein a dose amount of said lightly doped region is 1×10¹³ to5×10¹⁴ atoms/cm².
 20. A method according to claim 15 wherein saidinterlayer insulating film comprises silicon oxide.
 21. A methodaccording to claim 15 wherein said irradiating step is performed withoutmoving said laser light at a region of said driver circuit.
 22. A methodaccording to claim 15 wherein said irradiating step is performed so thatone portion of said semiconductor film is irradiated with 10 pulses ofsaid laser light.
 23. A method of manufacturing a display device havingan active matrix circuit and a driver circuit both formed over a samesubstrate, said driver circuit including at least one CMOS devicecomprising an N-channel type thin film transistor and a P-channel typethin film transistor, said method comprising the steps of: forming asemiconductor film comprising silicon over said substrate; irradiatingsaid semiconductor film with a laser light wherein an irradiation areaof said laser light on said semiconductor film is elongated in onedirection; etching the semiconductor film after the irradiating stepinto at least one semiconductor island; and introducing an N-typeimpurity into portions of said semiconductor island to form source anddrain regions and at least one lightly doped region for said N-channeltype thin film transistor, wherein said P-channel type thin filmtransistor has no lightly doped region.
 24. A method according to claim23 wherein said semiconductor film comprises crystalline silicon.
 25. Amethod according to claim 23 further comprising a step of annealing thesemiconductor island in a hydrogen atmosphere after the introduction ofthe N-type impurity.
 26. A method according to claim 23 wherein saidlaser is a pulsed excimer laser.
 27. A method according to claim 23wherein a dose amount of said lightly doped region is 1×10¹³ to 5×10¹⁴atoms/cm².
 28. A method according to claim 23 wherein said irradiatingstep is performed without moving said laser light at a region of saiddriver circuit.
 29. A method according to claim 23 wherein saidirradiating step is performed so that one portion of said semiconductorfilm is irradiated with 10 pulses of said laser light.
 30. A method ofmanufacturing a display device having an active matrix circuit and adriver circuit both formed over a same substrate, said driver circuitincluding at least one CMOS device comprising an N-channel type thinfilm transistor and a P-channel type thin film transistor, said methodcomprising the steps of: forming a semiconductor film comprising siliconover said substrate; irradiating said semiconductor film with a laserlight wherein an irradiation area of said laser light on saidsemiconductor film is elongated in one direction; etching thesemiconductor film after the irradiating step into at least onesemiconductor island; and introducing an N-type impurity into portionsof said semiconductor island to form source and drain regions and atleast one lightly doped region for said N-channel type thin filmtransistor, wherein said lightly doped region is formed only in saidN-channel type thin film transistor.
 31. A method according to claim 30wherein said semiconductor film comprises crystalline silicon.
 32. Amethod according to claim 30 further comprising a step of annealing thesemiconductor island in a hydrogen atmosphere after the introduction ofthe N-type impurity.
 33. A method according to claim 30 wherein saidlaser is a pulsed excimer laser.
 34. A method according to claim 30wherein a dose amount of said lightly doped region is 1×10¹³ to 5×10¹⁴atoms/cm².
 35. A method according to claim 30 wherein said irradiatingstep is performed without moving said laser light at a region of saiddriver circuit.
 36. A method according to claim 30 wherein saidirradiating step is performed so that one portion of said semiconductorfilm is irradiated with 10 pulses of said laser light.
 37. A method ofmanufacturing a display device having an active matrix circuit and adriver circuit both formed over a same substrate, said driver circuitincluding at least one CMOS device comprising an N-channel type thinfilm transistor and a P-channel type thin film transistor, said methodcomprising the steps of: forming a semiconductor film comprising siliconover said substrate; irradiating said semiconductor film with a laserlight wherein an irradiation area of said laser light on saidsemiconductor film is elongated in one direction; etching thesemiconductor film after the irradiating step into at least onesemiconductor island; introducing an N-type impurity into portions ofsaid semiconductor island to form source and drain regions and at leastone lightly doped region for said N-channel type thin film transistor;and forming an interlayer insulating film over said semiconductorisland, wherein said P-channel type thin film transistor has no lightlydoped region.
 38. A method according to claim 37 wherein comprisescrystalline silicon.
 39. A method according to claim 37 furthercomprising a step of annealing the semiconductor island in a hydrogenatmosphere after the introduction of N-type impurity.
 40. A methodaccording to claim 37 wherein said laser is a pulsed excimer laser. 41.A method according to claim 37 wherein a dose amount of said lightlydoped region is 1×10¹³ to 5×10¹⁴ atoms/cm².
 42. A method according toclaim 37 wherein said interlayer insulating film comprises siliconoxide.
 43. A method according to claim 37 wherein said irradiating stepis performed without moving said laser light at a region of said drivercircuit.
 44. A method according to claim 37 wherein said irradiatingstep is performed so that one portion of said semiconductor film isirradiated with 10 pulses of said laser light.